Originally Posted by Prism99

All patterns are drafted with standard 1/4" seams. The problem comes with actually achieving that standard. If we sew an exact 1/4" seam and then press, the seams takes up a little more than 1/4" because of turn-of-the-cloth and thread width. This slight difference doesn't matter in simple block patterns, but starts creating havoc when the quilter needs to match up complex blocks with one another.

The goal is to achieve an accurate 1/4" seam *after pressing* so that the block measures correctly. To achieve that, it's necessary to sew slightly less than 1/4" seam.